With the rapidly increasing number of workstations and personal computers (e.g. desktop or handheld ones) in all areas of business, administration, fabrication etc., there is also an increasing demand for flexible and simple interconnection of these systems. There is a similar need as far as the hook-up and interconnection of peripheral devices, such as keyboards, computer mice, printers, plotters, scanners, displays etc., is concerned. The use of electrical wire networks and cables becomes a problem in particular with increasing density of systems and peripheral devices and in the many cases where the location of systems, or the configuration of subsystems, must be changed frequently. It is therefore desirable to use wireless communication systems for interconnecting such devices and systems to eliminate the requirement of electrical cable networks.
In particular the use of infrared signals for exchanging information between systems and remote devices received increased interest during recent years. The advantage of such wireless infrared communications systems is the elimination of most of the conventional wiring. With respect to radio frequency (RF) wireless transmission, optical infrared (IR) wireless transmission has the advantages that no communication regulations apply and no PTT or FCC license is required. Additionally, no disturbance by electro-magnetic interference and no interference from other RF channels can occur, and the radiation is confined to a room so that better data security is guaranteed than with RF systems. There is thus no interference with similar systems operating next door and a higher degree of data security is afforded than radio-frequency transmission can offer. In contrast to radio-frequency antennae, the dimensions of light emitting diodes (LED) and photodiodes are usually smaller, which is of particular interest when designing portable computers.
Given an optical channel where the optical transmitter power, the ambient light conditions, and the photodiode area are all fixed quantities, power efficiency becomes the main criterion in choosing a modulation scheme in order to maximize the distance over which reliable transmission can be maintained. Judged by the power efficiency and ignoring bandwidth efficiency, L-slot pulse-position-modulation (L-PPM) would be the preferred modulation scheme for optical communication. Being a baseband modulation scheme, L-PPM is not suited for those applications where multiple collocated optical networks are required, since only a single L-PPM system can operate in a given location without coordination between collocated networks. As already mentioned, there is an increasing demand for optical wireless local area networks (WLANs) and peer-to-peer networks which can coexist independently within the same location. PPM is not suited for these kind of collocated networks.
There is no approach known which satisfies the requirements for high power efficiency and reliability under adverse conditions as well as the demand for collocated independent disturbance- and interference-free optical networks. While L-PPM might be the preferred method with respect to power efficiency, it cannot provide for
collocated multiple channels. PA1 duplex transmission, and PA1 flexible use and adaptation of bandwidth and data throughput.
In addition, optical PPM communications systems suffer from interference caused by fluorescent light sources at frequencies up to 500 kHz.
Frequency Shift Keying (FSK), on the other hand, is a carrier (bandpass) modulation scheme which is well suited for multiple channel operation but is poor in terms of power efficiency when compared to L-PPM. IR communications systems with bandwidths of up to 30 MHz can be achieved with todays component technology and further advances in terms of available bandwidth are expected in the future. Present baseband PPM systems do not fully exploit this available frequency spectrum.
It is another disadvantage of known optical communication systems that they are susceptible to interference caused by residual ambient light. Especially wireless IR data communication systems operating in a daylight environment are exposed to a high level of shot noise caused by light falling onto the receiver photodiode. Light from an incandescent desk lamp falling on the receiver diode is a source of shot noise having a detrimental influence on the IR communication, too. Due to residual ambient light, or due to the degree of optical path obstructions, the maximum transmission range between any two stations of an optical wireless communication system is variable. As a result, the network connectivity is unreliable except for very short distances.
Furthermore, the output power of a light source used as transmitter, e.g. a conventional LED or a laser diode, is limited such that the maximum transmission range is limited, too. These diodes can usually not be operated constantly, since the operation at high power has a detrimental influence on their lifetime. When increasing the photodiode area of the receiver, the transmission range may be further increased. Such a receiver is usually integrated in a portable computer or a peripheral device and the size of the photodiode area is limited due to design considerations and cost restrictions.